Piezoelectric ceramics have been widely used not only in a field of electronic devices such as resonators and filters, but also in products and the like such as sensors and actuators that use electric charge and displacement.
The conventional piezoelectric ceramics are generally ferroelectrics having a perovskite structure, such as lead titanate zirconate of the tetragonal system or trigonal system (PbZrO3-PbTiO3 solid solution, hereinafter referred to as PZT), or lead titanate of the tetragonal system (PbTiO3, hereinafter referred to as PT). By adding secondary components to these materials, there have been obtained ones having various piezoelectric properties.
However, many of these PZT-based or PT-based piezoelectric ceramics have Curie points of about 200 to 400° C. and thus become paraelectrics at higher temperatures to lose the piezoelectric properties thereof so that they can not be applied to usage at high temperatures, for example, nuclear reactor control sensors and the like. Further, the foregoing PZT-based or PT-based piezoceramics contain about 60 to 70 wt % lead oxide (PbO) and therefore are not preferable also in terms of ecology and prevention of environmental pollution.
In order to meet such a demand, there is disclosed, as piezoelectric ceramics having a high Curie point and containing no lead oxide at all, for example, a piezoelectric ceramic element using a piezoelectric ceramic composition containing SrBi4Ti4O15 as a main component and further containing at least one of Sc and Y in a range of 0.1 mol or less relative to 1 mol Bi in the main component (Unexamined Patent Publication No. 2001-172078).
Further, there are disclosed piezoelectric ceramics composed of a bismuth layer-structured compound containing (SrxLn1-x)Bi4Ti4O15 crystals (Unexamined Patent Publication No. 2000-143340), and piezoelectric ceramics composed of a bismuth layer-structured compound containing MIIBi4Ti4O15 crystals (MII is an element selected from Sr, Ba, and Ca) (Unexamined Patent Publication No. 2001-192267).
Here, in case of a resonator, since it is used as an inductor, there are required piezoelectric ceramics in which Qm (mechanical quality factor), one of important properties in the piezoelectric properties, or Qmax (maximum value of Q=tan θ, θ:phase) between a resonant frequency and an antiresonant frequency, is large.
However, with respect to the piezoelectric ceramic element disclosed in Unexamined Patent Publication No. 2001-172078, there has been a problem that although an electromechanical coupling coefficient kt and a resonant frequency temperature change rate frTC from −20° C. to 80° C. are improved, the foregoing Qmax is insufficient and therefore it does not have the sufficient piezoelectric properties applicable to the resonator.
On the other hand, the piezoelectric ceramics disclosed in Unexamined Patent Publication No. 2000-143340 or Unexamined Patent Publication No. 2001-192267 have large Qmax, but demands have been made for ones having larger Qmax as piezoelectric ceramics using thickness longitudinal vibration.